750ppm would be enough to sustain green growth provided it's balanced and providing enough Nitrogen. Your pH dropping suggests to me that you have another problem. Usually a drop in pH signals the start, or the acceleration of root disease. Upping the ppm is a good idea, dont get me wrong. But I would examine your roots carefully, if you have some turning a reddish/brown color, you need to get some bennies in there quickly.
Also, a good rule of thumb for flowering, is 100ppm increase on week two through week 6 up to a maximum of 1600ppm, if your strain can tolerate it. I usually start week one with 1000ppm, by week 6 I'm up to 1400 or so. I like to error on the side of caution. Plants are not cow's, they cannot be force fed, so concentration is not as important as balance among the macro/micro nutrients. By increasing the salt (nutrient) content you increase the osmotic pressure, hence allowing the plant to do less work to take up the nutes. But on the flip side, too high, and the osmotic pressure inside the plant rises to the point to where it can no long assimilate water and nutrients. Thus causing the plant to uptake very little of either, and without rectifying the situation, you end up with nute burn....
If I were you, and seeing such an unusual dropping of leaves, I would start by completely cleaning your res, give fresh nutes, add EWC tea, and wait a few days, problem should be fixed, if not, it most likely isnt your water.....
